The present invention relates to electrically conductive composite materials and more particularly to apparatus and a method for increasing the electrical conductivity of composite materials and to the products of such method. While the invention relates primarily to solid or foamed resinous composites, it is believed that the principles of the invention may also be applicable to materials other than resinous materials including various other high resistivity composite material matrices such as ceramics, wood, concrete and plaster, for example.
Conductive resinous materials are well-known in the art and have been used in a variety of applications as replacements for metals and in applications uniquely adapted for plastics.
Two basic approaches are known in producing conductive resinous materials such as plastics: coating or otherwise laminating a conductive surface or layer abutting the plastic surface and impregnating the plastic with conductive materials, forming a plastic composite.
Low restivities of from 0.01 to 1 ohm-cm have heretofore required the application of high conductivity paint coatings such as silver, the gluing of metal foils, or the plating of metal layers on the plastic surfaces. Such approaches are disadvantageous from several standpoints. The cost of the metal, particularly silver, is high as is the cost of application or deposition of the metal coating or plating. Furthermore, the resulting product has a fragile surface unsuitable for many applications. In addition, the interconnection of panels so as to maintain overall conductivity is difficult.
The impregnation of resinous materials by adding quantities of metal powders, carbon powders, metal fibers (including ribbons and other shapes) or metallized glass fibers can provide resistivities in the order of 0.01 ohm-cm to 1,000 ohm-cm if conductive material from the order of 5-80% by weight is used. Generally, the lower resistivity requires a higher proportion of conductive material which not only increases the cost of product but also may degrade the physical properties of the plastic. In addition, the use of carbon, for example, may limit the available color to black or other undesirable colors.